This invention generally relates to the field of seismic prospecting. More particularly, this invention relates to a method for attenuating noise from seismic data.
Marine seismic exploration usually involves acquiring seismic data using a seismic acquisition system whose source initiates a down-going seismic wavefield. A portion of the down-going wavefield travels into the underlying earth where it illuminates subsea geologic formations. As it illuminates the interfaces or boundaries between the formations, part of the wavefield is returned (or reflected) back through the earth (propagating in the up-going direction). Part of the reflected wavefield is received by the seismic acquisition system, converted into electrical signals, and recorded for subsequent processing. An analysis of these recorded signals makes it possible to estimate the structure, position, and lithology of subsea geologic formations, thereby completing an important step in the exploration process.
FIG. 1 shows a simplified example of a typical marine seismic acquisition system. A first ship 1 tows a seismic source 2 several feet below the surface 3 of the ocean. The seismic source 2 is activated to produce a down-going wavefield 4d that is at least partially reflected by a subsea formation boundary (i.e., an impedance discontinuity) 5. The up-going wavefield 4u then travels toward the sensors 6 and is detected.
The sensors 6 used in typical marine seismic exploration include pressure sensors and velocity (also referred to as xe2x80x9cparticle velocityxe2x80x9d) sensors. Typically, the pressure sensors are hydrophones and the velocity sensors are geophones. The hydrophones measure a scalar pressure and are not sensitive to the propagation direction of the wavefield. The geophones, which may be vertical geophones, provide vector response measurement whose polarity depends on whether the direction of propagation of the wavefield is up-going or down-going. The amplitude of the geophone response is also related to an angle of the propagation relative to the sensitive direction of the geophone. If a wavefield is recorded by a hydrophone and a vertical geophone having the same electronic impulse response, then a polarity comparison between the hydrophone and geophone measurement determines whether the wavefield is propagating in the up-going or down-going direction. Hydrophones and geophones disposed at the seafloor are typically used in pairs when collecting seismic data.
A pressure and vertical velocity combination from two-component (2C) or from multi-component (typically 4C) has been useful to cancel down-going multiples from a combined pressure and vertical velocity data signal. Noise that is present on one or the other of these sensors can limit the effectiveness of the combination technique. This invention attenuates noise that is normally present during data acquisition and has been found to be an important processing step before the combination.
In one type of marine seismic surveying, the sensors 6 are located at regular intervals in one or more ocean bottom cables (OBC) 7 that are arranged on the seafloor 9. When necessary, a second ship 8 is used to move the OBC 7 to a new position on the seafloor 9. Several miles of OBC 7 are typically deployed along the seafloor 9, and several OBCs are typically deployed in parallel arrangements. OBC 7 arrangements are particularly well suited for use in certain zones (i.e., zones cluttered with platforms or where the water is very shallow) where the use of ship-towed hydrophone arrays (not shown) (which are located proximate to the ocean surface 3 and are typically referred to as xe2x80x9cstreamersxe2x80x9d) is not practical. In another type of seismic surveying, the hydrophones and geophones are deployed on the ocean bottom as separate ocean bottom stations (OBS). A combination of separate OBS and OBC can be deployed. The geophone and hydrophone must be connected to a recording instrument typically on a vessel.
As shown in FIG. 2, in another type of marine seismic surveying, a marine cable or streamer 21 incorporating pressure hydrophones are designed for continuous towing through the water. A marine streamer 21 is typically made up of a plurality of active or live hydrophone arrays 23 separated by spacer or dead sections 25. Usually the streamers are nearly neutrally buoyant and depth controllers or depressors 27 are attached to depress the streamer 21 to the proper towing depth. A tail buoy with a radar reflector 29 is typically attached to the end of the streamer. The entire streamer may be 3-6 Km in length and is towed by a ship 31.
Seismic energy is transmitted by xe2x80x9cbody waves,xe2x80x9d which can be either compressional waves (P-waves) or shear waves (S-waves). P-waves are elastic solid waves in which particle motion is in the direction of propagation. S-waves are body waves in which the particle motion is perpendicular to the direction of propagation. Seismic energy can also be transmitted along boundaries between substances having different elastic properties by xe2x80x9csurface waves.xe2x80x9d
Claerbout has disclosed that there is xe2x80x9ccrosstalkxe2x80x9d between vertical and horizontal geophones due to non-vertical incidence of P and S reflected waves at the water bottom (Claerbout, J., The Leading Edge, v. 9, No. 4, pp. 38-40, April 1990). Crosstalk occurs when the vertical geophone records at least part of the S-wave energy. This energy becomes noise because the combination processing step assumes only P-waves. Other researchers have attributed this noise to non-perfect coupling and orientation of OBC geophones (Li, X. Y. and Yuan, J., Geophysical Prospecting v 47, No. 6, pp 995-1013, November 1999). It has also been suggested that this noise will be eliminated when the vector fidelity problem for OBC geophones is solved with the next generation of OBC hardware (Reid, F. and Macbeth, C. 70th Annual SEG. Int. Mtg. Expanded Abstr. Biogr. v. 1, pp 1213-1216, 2000). In addition, all the prior art methods attribute the xe2x80x9cleakage noisexe2x80x9d in the vertical geophone to shear waves, thus they do not consider other possible sources of the coherent noise (e.g., surface related waves).
In general, the industry recognizes that OBC vertical geophone data is typically noisy. FIG. 3(a) shows seismic data traces collected using a hydrophone on an ocean bottom cable. FIG. 3(b) shows the corresponding geophone seismic traces collected using a vertical geophone on the same ocean bottom cable. As can be seen in FIGS. 3(a) and 3(b) there is significantly less noise from the seismic data traces collected with the hydrophone.
Practical attempts to suppress this noise usually include considering the acquisition system (i.e., vector fidelity, coupling, orientation) and then treating the noise as a random noise in the common-shot domain or common-shot order. There is lack of successful examples of noise suppression examples in real OBC seismic data. The lack of understanding on how to attenuate this noise has led to noise contaminated results of 2C OBC data combination worldwide. In some parts of the world (e.g., Gulf of Mexico), the noise is so severe that 2C OBC data is often considered to be useless. Accordingly, there is a need to attenuate, efficiently filter, or suppress at least part of this noise from OBC data. The present invention satisfies this need.
One aspect of the invention is a method for attenuating noise in seismic data traces, at least a portion of the noise is noncoherent in the common-shot domain but coherent in the common-receiver domain. The coherent noise has a move-out velocity different from the move-out velocity of the data signal in the seismic data traces. The method comprises (a) sorting the seismic data traces into common-receiver order; and (b) using the difference in move-out velocities to separate the coherent noise from the data signal.
In another aspect, the invention is a method for attenuating noise in two-component or multi-component ocean bottom cable (OBC) seismic data traces. The OBC seismic data traces comprises hydrophone data traces and vertical geophone data traces, at least a portion of the noise in the data traces being noncoherent in the common-shot domain but coherent in the common-receiver domain. The coherent noise has a move-out velocity different from the move-out velocity of the data signal in the seismic data traces. The method comprises: (a) sorting the data traces into common-receiver order; (b) applying a move-out correction to the data traces using a move-out velocity chosen to overcorrect the data signal and undercorrect the coherent noises; (c) transforming the move-out corrected data traces from the x-t domain to a two-dimensional domain in which the move-out corrected data signal and the move-out corrected noise are separated; (d) removing at least a portion of the coherent noise from the transformed data traces; and (e) inverse transforming the data traces from the two-dimensional domain back to the x-t domain. Furthermore, an inverse moveout may be performed after the traces are transformed back to x-t domain with the velocity used in step (b).